Maybe it is not SQL Developer itself however here are the steps that I can reproduce it.

Open a report in Developer and set it to refresh every 5 seconds.

Open notepad on my other monitor and start typing random characters.

When the report refreshes I lose focus on notepad but it does not appear that SQL Developer gets that focus more like I just lose the focus on the current window. Wonder if its task bar related and not directly SQL Developer causing it.

Just checked the SQL Developer window does get the focus. The title bar changes like its active.